The present invention relates to systems and methods for analyzing tissue structure and function. More particularly, the invention relates to a method for determining tissue wall viscoelasticity, such as viscoelasticity of a bladder wall, using ultrasound vibrometry to excite mechanical waves in the bladder wall and track the motion using ultrasound pulse-echo techniques.
An elastic bladder is important for storing increasing volumes of urine at low pressures. Increases in bladder elasticity (bladder stiffening) resulting in bladder dysfunction are associated with age and pathogenic processes, and are largely due to an increase in connective tissue relative to the smooth muscle in the bladder wall. A variety of congenital conditions such as urethral defects and neuropathic conditions (i.e. myelomeningocele), and conditions such as spinal cord trauma, tumors, or obstructive uropathy are associated with bladder dysfunction. As the percent of connective tissue increases compared to smooth muscle, the bladder becomes more rigid, and is less able to expand during filling. This can result in decreased bladder capacity, which may manifest in lower urinary tract symptoms, including incontinence. Reduced bladder compliance (defined as changes in bladder volume due to a change in the detrusor muscle pressure) is of particular importance in subpopulations of patients with neurogenic bladder disorders who must be carefully monitored with urodynamic studies (UDS) on an annual basis to ensure that bladder pressures stay within normal parameters.
Urodynamic studies are currently considered the gold standard in clinical assessment for measuring bladder compliance, but are uncomfortable and carry risks of infection. A typical procedure lasts approximately 45 minutes, requires catheter placement in the bladder and the vagina or rectum, filling the bladder at a defined rate, and measuring the change in Pdet defined as detrusor pressure as the bladder fills. A compliant (elastic) bladder expands to accommodate the filling volume, resulting in a low detrusor pressure, while a non-compliant (inelastic) bladder does not expand as readily and the detrusor pressure rises during bladder filling. The urodynamic studies are often accompanied by pain during and following the procedure, traumas and infection due to urinary catheterization have been reported in both men and women. In addition to discomfort and clinical risks, urodynamic studies are labor and resource intensive for the institutions that treat and follow these patients.
Several ultrasound-based studies have noted that the bladder wall thickness (BWT) and total bladder weight increases in animal models of intravesical obstruction. Increases in BWT have been attributed to increased fibrosis in the bladder as the result of aging or pathogenic processes. Increased thickening of the bladder wall corresponds to increases in bladder weight, both of which may correspond to a reduction in bladder elasticity and decline in bladder function. Ultrasound methods have therefore been adapted to measure BWT, bladder surface area, and total estimated bladder weight, and are currently being examined as potential non-invasive measures of bladder structure and function. These ultrasound measurements of bladder weight and wall thickness provide indirect measures of structural changes. That is, these methods rely on metrics, such as BWT, bladder surface area, and bladder weight as metrics that can be indirectly correlated with bladder functionality or structure. Regardless of the cause of dysfunction, the structure, thickness and biomechanics of the bladder wall can be altered significantly. However, these and other available techniques do not provide a non-invasive and relatively pain-free, quantitative measurement of bladder elasticity. Instead, in clinical practice, clinicians must rely on urodynamic studies despite the above-described risks and drawbacks.
Therefore, there is a need for systems and methods to non-invasively and painlessly provide salient information regarding the mechanical properties of the bladder wall in patients who experience bladder dysfunction.